1. Field of the Invention
The present invention relates to a polymer electrolyte, particularly, a polymer electrolyte suitably used for a fuel cell and a method for producing the same.
2. Description of the Related Art
As a proton conductive polymer electrolyte used in a solid polymer-type fuel cell, perfluorosulfonic acid-based materials typically including Nafion (trade mark of Dupont) are mainly used conventionally because of their excellent property as a fuel cell.
However, this material is very expensive which is believed to be a serious problem in wide-spreading a powder generation system using a fuel cell in the future.
Under such circumstances, there are recently activating developments of cheap polymer electrolytes which can replace the perfluorosulfonic acid-based materials. Among others, materials obtained by introducing sulfonic acid groups into an aromatic polyether which is excellent in heat resistance and has high film strength are recognized as hopeful, and for example, JP-A (Kohyo publication) No. 11-502249 describes a sulfonated polyether ketone-based polymer electrolyte, and JP-A Nos. 10-45913 and 10-21943 describe a sulfonated polyether sulfone-based polymer electrolyte.
In these materials, when the amount of introduced sulfonic acid group is larger, proton conductivity increases generally, however, water absorption of a polymer tends to increase simultaneously. When a film produced from a polymer having high water absorption property is used in a fuel cell, water produced in using the cell causes large dimension change and reduction in strength.
On the other hand, JP-A (Kohyo publication) No. 10-503788 describes a polymer electrolyte obtained by introducing sulfonic acid groups into a styrene-(ethylene-butylene)-styrene triblock copolymer. There is a fear of low heat resistance of this block copolymer since the main chain of the copolymer is flexible.